Advances in Meteorology
Volume 2019 (2019), Article ID 1853797, 15 pages
https://doi.org/10.1155/2019/1853797

Research Article

Analysis of the Gale in the Bohai Sea Caused by Tropical Cyclone “Yagi”

Tiantian Hu,¹ Binxian Liu,¹ Di Wu,² and Xiaoyuan Yi³

¹Tianjin Central Observatory for Oceanic Meteorology, Tianjin Meteorological Bureau, Tianjin 300074, China
²Department of Aviation Meteorology, College of Air Traffic Management, Civil Aviation University of China, Tianjin 300300, China
³Tianjin Meteorological Observatory, Tianjin Meteorological Bureau, Tianjin 300074, China

Correspondence should be addressed to Binxian Liu; binxian_l@163.com

Received 13 April 2019; Revised 17 July 2019; Accepted 21 July 2019; Published 28 August 2019

Academic Editor: Alastair Williams

Copyright © 2019 Tiantian Hu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

In this study, we analyzed the meteorological processes associated with 2018 tropical cyclone No. 14, “Yagi.” TC Yagi continued moving northeastward after losing its numerical designation from the National Meteorological Center of the China Meteorological Administration (CMA) because of weakening and then restrengthened when it moved over the Bohai Sea, inducing an ocean gale on 14-15 August 2018. The results of our investigation revealed that the continued northeastward movement of Yagi on 14 August was related to the divergence of the upper-level westerly jet stream, the northward shift of the subtropical high in the midtroposphere, as well as the steering flow and asymmetrical air flow around the disturbance itself in the lower troposphere. The enhancement of Yagi over the Bohai Sea on the night of 14 August was related to the decrease of friction over the ocean and the increase of diabatic heating from the sea surface flux. The wind speed increased to a maximum when the depression moved over the Bohai Sea, an occurrence that was not only due to the enhancement of the cyclone itself but also due to the flow of cold air from high latitudes along the north side of the Bohai Sea. The behavior of the cold air was related to the shift of the convergence zone in the upper-level westerly jet at 200 hPa, long-wave troughs and ridges at 500 hPa, and terrain effects. Thus, the gale development in the Bohai Sea was due to both the enhancement of tropical cyclone Yagi after it moved over the ocean and the flow of cold air from high latitudes.

1. Introduction

Tropical cyclones (TCs), including typhoons, tropical storms, and tropical depressions, are strong, warm-core atmospheric vortices that develop over the tropical ocean [1]. TCs are always accompanied by strong gales, heavy rainfall, and storm surge [2]. China is one of the countries that are most seriously impacted by TCs, with an average of 7-8 TCs making landfall there each year [3]. TCs are born in the Northwest Pacific Ocean and are classified into two categories based on their track: one type moves west-northwestward into the South China Sea or the East China Sea, making landfall in either the Philippines, Vietnam, or China, and the other type moves northwest along a parabolic track, suddenly turning northward towards the north coast of China [4, 5]. TC intensity is classified into six levels by the China Meteorological Administration (CMA), as listed in Table 1.

Table 1: TC intensity levels as defined by the CMA.

TC type
Maximum average wind speed near the surface of the storm center (m/s)

Tropical depression
10.8–17.1

Tropical storm
17.2–24.4

Severe tropical storm
24.5–32.6

Typhoon
32.7–41.4

Severe typhoon
41.5–50.9

Super typhoon
≥51.0

Research has revealed that there are many factors which affect the development and movement of TCs, including sea surface temperature [6–8], ambient airflow, the β effect [9], the internal TC structure, terrain, and the interaction between multiscale weather systems [10–13]. There are three dynamic factors involved in TC movement: external forces, internal processes, and the interaction between them [14–16]. The tracks of TCs are primarily determined by the environmental air flow [17, 18]. Large-scale environmental steering flow is the domination external influence on a TC, accounting for 70–90% of a TC movement [19, 20]. The structure of TCs and their atmospheric flow field determine that TCs born in the Northwest Pacific Ocean generally move northwestward [21, 22]. The western Pacific subtropical high (WPSH), the westerly trough, and the internal force of the TCs themselves are the main influence factors of the tracks of TCs that are born in the western Pacific Ocean. Wang et al. [23] considered that the anticyclonic flow field of the WPSH directly affects seven possible TC paths, which include moving northward along the coast of East Asia, making landfall in East China, making landfall in South China, moving westward, turning out to sea after landfall, veering in the offshore, and veering in the open sea. Furthermore, westerly flow, especially the distribution and variation of the westerlies in the upper levels of midlatitudes and high latitudes, exerts significant influence on the characteristics of various TC tracks. In addition, the interactions between synoptic-scale vortices and TCs [24], as well as interactions of a TC with other TCs, also have important effects on TC tracks [25]. The Bohai Sea and the Yellow Sea constitute the northern ocean of China. The gales and storm surges caused by northward-moving TCs are the most serious summertime meteorological disasters in that region. For this reason, many meteorologists have investigated the mechanisms of northward-moving TCs [26–29]. The structure of a TC can be described by a number of parameters associated with the TC wind [30]. Intensity (i.e., the maximum sustained surface wind) is critically important because the wind-related damage of a TC is proportional to at least the square of the maximum surface wind. The gale processes are characterized by intense pressure gradients and strong winds such as northeasters, thunderstorms, and frontal systems that bring similar types of weather patterns [31].

This study analyzed the meteorological processes associated with the gale in the Bohai Sea that was caused by tropical depression Yagi on 14-15 August 2018. In addition, the effects of the WPSH, upper-level trough, jet stream variability, and sea surface flux on the track and intensity changes of Yagi were investigated. This manuscript is organized as follows: Section 2 describes the data and methods. Section 3 describes the observed features of the track of TC Yagi and the induced gale process in the Bohai Sea. Section 4 investigates the development mechanism of tropical depression Yagi in terms of atmospheric circulation. Section 5 includes the model simulation and two sensitivity experiments used to explain the effects of sea surface change on the development of Yagi. Our conclusions and discussion are presented in Section 6.

2. Data and Methods

The best-track dataset of TC Yagi from the China Meteorological Administration (CMA) website (http://data.cma.cn) was used in this study. In order to analyze the gale in the Bohai Sea which was caused by Yagi on 14-15 August 2018, the hourly observed data from two automated weather stations on two platforms in the Bohai Sea were used, including wind speed and wind direction data. Visible cloud imagery from the FY2 satellite was also utilized to determine the intensity of tropical depression Yagi once it had moved over the Bohai Sea.

In order to analyze the characteristics of the atmospheric circulation of Yagi, 6-hourly daily reanalysis data from the European Centre for Medium-Range Weather Forecasts Interim (ERA-Interim) product were used. The horizontal resolution of this dataset is 0.25° × 0.25° [32].

Tropical depression Yagi reversed its track after weakening on 14 August. The northeastward curving of Yagi took it over the Bohai Sea, where it strengthened. Thus, the steering flow was a key factor in the track of Yagi [18, 20, 33]. In this study, we computed the steering flow by vertically integrating the pressure-weighted flow from 850 to 200 hPa [20].

To further illustrate the effects of the sea surface flux on the development of tropical depression Yagi, the apparent heat source was computed [34]. The apparent heat source (Q₁) can be calculated as

where is the temperature; is the horizontal velocity vector; is the air pressure; is the vertical pressure velocity in units of Pa·s⁻¹; is the potential temperature; is the reference air pressure of 1000 hPa; and is a constant, with a value of 0.286.

The spatial intensity variation of can reflect the diabatic heating change [34].

In order to examine the effects of friction and surface flux change on tropical depression Yagi when it moved from land to sea, we used the Weather Research and Forecasting (WRF) mesoscale model version 3.9.1 to simulate this process. The two-layer nesting scheme was used for simulation (Figure 1(a)). The outer layer included mideastern China and the Northwest Pacific, while the inner layer included North China as well as the Bohai Sea and Yellow Sea. The parameter settings used in the simulation, as found in Peng et al.’s study [35], are listed in Table 2.

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Figure 1: (a) Simulation domain; (b) land use categories in the control experiment (CE); (c) land use categories in sensitivity experiment 1 (SE_1). The numbers on the legend bar represent the 20 MODIS land use categories modified by the IGBP (1: evergreen needleleaf forest; 2: evergreen broadleaf forest; 3: deciduous needleleaf forest; 4: deciduous broadleaf forest; 5: mixed forest; 6: closed shrubland; 7: open shrubland; 8: woody savanna; 9: savanna; 10: grassland; 11: permanent wetland; 12: cropland; 13: urban and built-up land; 14: cropland/natural vegetation mosaic; 15: snow and ice; 16: barren or sparsely vegetated; 17: water; 18: wooded tundra; 19: mixed tundra; 20: barren tundra).

Table 2: Simulation scheme.

Term
Scheme

Central position
39°N, 122°E

Horizontal resolution
27 km, 9 km

Time step
90 s

Boundary layer scheme
YSU scheme

Land surface scheme
Standard Noah land surface model

The WRF model was run from 0600 UTC 14 August to 0000 UTC 16 August 2018. Since tropical depression Yagi entered the south Bohai Sea around 1200 UTC 14 August and made landfall again around 1200 UTC 15 August, the first 6 hours (from 0600 UTC to 1200 UTC 14 August) were used as the spin-up time, with the analysis beginning at 1200 UTC 14 August. The driving wind data were the 6-hour data of the National Centers for Environmental Prediction (NCEP), and the driving sea surface temperature data were the daily data of the National Oceanic and Atmospheric Administration (NOAA).

In order to analyze the effects of the underlying surface changes on the development of tropical depression Yagi, in the control experiment (CE), we ran the model without any changes. In sensitivity experiment 1 (SE_1), we changed the land use categories in the Bohai Sea and Yellow Sea from water to cropland in domain 2 (Figures 1(b) and 1(c)). The sea surface was changed to land in SE_1. In sensitivity experiment 2 (SE_2), we set isfflx = 0 in namelist.input so that the sea surface heat and moisture fluxes were turned off during the simulation. The intensity and track of Yagi were compared in the CE, SE_1, and SE_2.

3. Observational Characteristics

TC Yagi was born in the Northwest Pacific Ocean at 0600 UTC 7 August 2018 and made landfall on the East China coast at 1600 UTC 12 August (Figure 2(a)). Yagi had the intensity of a severe tropical storm when it made landfall. As Yagi continued moving northwestward, its intensity weakened to tropical storm and then to tropical depression. Tropical depression Yagi lost its TC designation number from the National Meteorological Center of the China Meteorological Administration (CMA) at 0000 UTC 14 August at 35°N, 116.1°E. Tropical depression Yagi then turned northeastward and continued moving. The center of the depression strengthened after it moved into the southwest side of the Bohai Sea at 1800 UTC 14 August. At approximately 1200 UTC 15 August, tropical depression Yagi again made landfall on the south coast of the Bohai Sea, where it then weakened and dissipated (Figure 2(a)).

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Figure 2: (a) Track of TC Yagi. The different-colored points represent different intensities. Green points represent Yagi after it lost its numerical designation; yellow points represent tropical depression strength; orange points represent tropical storm strength; red-orange points represent severe tropical storm strength. (b) Visible cloud image of Yagi from the FY2 satellite at 0400 UTC 15 August. (c) Terrain around the Bohai Sea. Points A and B represent the location of the 2 automated observation platforms.

During the daylight hours of 15 August, tropical depression Yagi strengthened after moving into the Bohai Sea, where it remained for an extended period of time and presented a distinct typhoon circulation structure in the satellite imagery (Figure 2(b)). The terrain around the Bohai Sea can be seen in Figure 2(c). During the daylight hours of 15 August, the maximum average hourly wind speed in the Bohai Sea reached as high as 33.1 m/s, almost matching the intensity of a severe tropical storm. Figure 3 shows the wind changes observed by the two platforms in the Bohai Sea. Platform A is in the center of the Bohai Sea, while platform B is in the northern portion of the Bohai Sea (the locations of the platforms are illustrated in Figure 2(c)). It can be seen that the wind speed strengthened between 0200 and 0600 UTC 15 August. The maximum wind speed observed by platform A was 31.7 m/s at 0400 UTC 15 August, while the maximum wind speed observed by platform B was 22.8 m/s at 0400 UTC 15 August. The wind direction at platform A was northeast, and the wind direction at platform B was northwest on the evening of the 14th, veering to the north and northeast during the night of the 14th and during the entire day of the 15th.

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Figure 3: Wind change from 0800 UTC 14 August to 0000 UTC 16 August 2018. The hourly data were observed at (a) platform A and (b) platform B.

4. The Development Mechanism of Tropical Depression Yagi

To analyze the development mechanism of tropical depression Yagi after its track turned northeastward, as well as the factors leading to the gale in the Bohai Sea, the ERA-Interim reanalysis data were utilized to determine the atmospheric circulation pattern. TC Yagi weakened to a tropical depression and lost its TC number at 0000 UTC 14 August 2018. At that time, the circulation center of the depression was located at 34.7°N, 116.1°E, over North China. Based on the mean sea level pressure and 10 m winds at 0000 UTC 14 August (Figure 4(a)), there was a high-pressure center at high latitudes (50°N, 110°E). The 1000–500 hPa thickness was also calculated in order to analyze the thermodynamic characteristics. It could clearly be seen that the high-pressure area had a cold air structure. With the eastward movement of the system, the high-pressure center gradually moved southeastward, thereby inducing the cold air to also move southeastward. At 1200 UTC 14 August (Figure 4(b)), the center of tropical depression Yagi had weakened. There were no closed isobar contours in the surface pressure field around Yagi. The circulation center was located over land, to the south of the Bohai Sea. The gale which was caused by the periphery of the circulation center of Yagi began to affect the central and western Bohai Sea. At the same time, as the high-pressure system at high latitudes moved southeast, the terrain effect of Baekdu Mountain caused the cold air to flow from the northeast over the north side of the Bohai Sea. During the interaction of Yagi with the cold air, the average wind speed strengthened sharply over the ocean, a common phenomenon with tropical cyclones. At 0000 UTC 15 August (Figure 4(c)), the center of tropical depression Yagi entered the Bohai Sea from the south, with the cyclone strengthening after its center moved over the sea surface and forming an apparent clear, warm circulation center, which is characteristic of TCs. The cold air from high latitudes expanded farther southward. The southern periphery of the high-pressure center and the northwest periphery of Yagi formed a tight pressure gradient zone, strengthening the wind speed in the zone, which induced a heavy gale in the Bohai Sea. At approximately noon (0400 UTC 15 August), the average wind speed reached its maximum. At 1200 UTC 15 August (Figure 4(d)), with the further expansion of the high-pressure system from high latitudes, the southward cold air pushed Yagi back over land on the south side of the Bohai Sea. This effectively destroyed the warm center structure of tropical depression Yagi, and its intensity weakened. At this point, there were no closed contours around Yagi in the surface pressure field. The pressure gradient was tight on the northwest side of Yagi and the southeast side of the high-pressure periphery, i.e., in the Bohai Sea area. The wind direction was northeast, and the average wind speed had slightly weakened over the Bohai Sea. With the further expansion of the cold air from high latitudes, tropical depression Yagi was pushed back over land, where it gradually dissipated. The meteorological processes leading to the gale over the Bohai Sea caused by Yagi came to an end.

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Figure 4: Surface circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The contours represent the mean sea level pressure (hPa), the vectors represent the 10 m wind speed (m/s), the shading represents the 1000–500 hPa thickness (m), and the red dots represent the location of “Yagi.”

From the geopotential height (GPH) and wind field at 200 hPa on 14-15 August 2018 (Figure 5), it can be seen that the westerly jet at high latitudes exhibited an obvious wave pattern. The meridional wind was distinct. At 0000 UTC 14 August (Figure 5(a)), obvious positive divergence existed above the Bohai Sea. This indicated that the upper-level jet stream was divergent in this region, which induced an updraft in the midlevels and low levels and convergence in the lower troposphere. This structure benefited tropical depression Yagi as it moved northeast into the Bohai Sea. Meanwhile, the convergence area of the upper westerly jet located at higher latitudes, which favored the formation of a downdraft, corresponded to the formation of a cold high-pressure system at the surface. At 1200 UTC 14 August (Figure 5(b)), the eastward movement of the convergence area of the upper westerly jet induced the cold high-pressure system at the surface to move east. The divergence area of the upper-level westerly jet at 200 hPa was still located above the Bohai Sea, which induced tropical depression Yagi to continue moving northeastward into the Bohai Sea. At 0000 UTC 15 August (Figure 5(c)), the convergence zone of the upper-level westerly jet still moved southeastward, and its intensity had increased, thereby inducing a stronger southeastward expansion of cold air at the surface. At 1200 UTC 15 August (Figure 5(d)), the long wave troughs and ridges of the upper-level westerlies continued to progress eastward, and the convergence area of the upper-level westerly jet now controlled the entire area above the Bohai Sea. Thus, the cold air had become the predominant factor on the surface of the Bohai Sea at that time, forcing tropical depression Yagi back over land.

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Figure 5: 200 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The contours represent the GPH (gpm), the vectors represent winds at 200 hPa, the shading represents the divergence at 200 hPa (×10⁻⁵·s⁻¹), and the red dots represent the location of “Yagi.”

From the circulation patterns at 500 hPa on 14-15 August (Figure 6), it can be seen that there were two troughs and one ridge at high latitudes. The meridional wind was obvious, and the location of the periphery of the subtropical high (588 gpm contour) was farther to the north. An obvious cyclone was present in the South China Sea, which was the severe tropical storm “Bebinca.” At 0000 UTC 14 August (Figure 6(a)), there was a trough east of eastern China at high latitudes. The trough was conducive to the pooling of cold air at the surface. The location of the periphery of the subtropical high was farther north than usual probably because of the effects of “Bebinca” on its southern side. This aided the continued northeastward movement of tropical depression Yagi along the western edge of this high. By 1200 UTC 14 August (Figure 6(b)), the system at high latitudes had moved eastward. With the progression of the trough, the cold air at the surface continued moving southeastward. The ridge of the subtropical high continued moving northward, exhibiting a zonal distribution that extended inland. This helped to steer Yagi northward into the Bohai Sea. At 0000 UTC 15 August (Figure 6(c)), the “two troughs and one ridge” configuration at high latitudes continued to move eastward. Through the effects of the trough at 500 hPa to the east, the cold air at low levels flowed into the Bohai Sea from the north along Baekdu Mountain. At this time, tropical depression Yagi strengthened after it moved over the sea. The location of the subtropical high was also farther north than usual, and the 588 line extended inland. The regions controlled by the subtropical high in the midlatitudes expanded. This induced tropical depression Yagi to remain over the sea for an extended period. At 1200 UTC 15 August (Figure 6(d)), with the eastward movement of the trough at 500 hPa, the southward expansion of the cold air, and the generation of TC No. 18 “Rumbia” in the East China Sea, the subtropical high was split, with its eastern portion soon appearing over the ocean. The southward expansion of cold air and the weakening of the subtropical high led to Yagi making landfall again, after which it soon weakened and dissipated.

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Figure 6: 500 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The blue contours represent the GPH (gpm), the red contours represent the temperature (K), and the red dots represent the location of “Yagi.”

The curving of tropical depression Yagi to the northeast had a significant impact on its redevelopment once it emerged over the ocean, resulting in a terrible gale in the Bohai Sea. The upper-level westerly jet as well as the subtropical high impacted the northeastward turning of Yagi. In addition, the steering flow was also an important mechanism in the curving of its track [20] (Figure 7). At 0000 UTC 14 August (Figure 7(a)), the steering flow was northward in the environment surrounding Yagi, inducing the depression to continue moving northward toward the Bohai Sea. At 1200 UTC 14 August (Figure 7(b)), Yagi was close to the south side of the Bohai Sea, embedded in a decidedly northeast steering current. This forced tropical depression Yagi into the Bohai Sea from its southwest coast. At 0000 UTC 15 August (Figure 7(c)), Yagi was in the Bohai Sea, and the steering flow around it had weakened, allowing the depression to remain over the Bohai Sea for an extended period. At 1200 UTC 15 August (Figure 7(d)), the steering flow around Yagi had turned to the southwest, pushing the depression back over land.

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Figure 7: Steering flow (×10² m/s): (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The black dots represent the location of “Yagi.”

The steering flow was the vertical integration of the pressure-weighted flow from 850 to 200 hPa [20]. In order to further explore the steering flow pattern, we generated the 700 hPa circulation patterns to reveal the effects of the low-level jet. From the circulation patterns at 700 hPa (Figure 8), at 0000 UTC 14 August (Figure 8(a)), a pronounced asymmetric structure existed on the east and west sides of tropical depression Yagi. The southerly airflow on the east side of the depression was noticeably stronger than the northerly airflow on its west side. The stronger southerly airflow induced the depression to move northward. The southerly airflow on the east side of the depression then shifted to southwesterly and was now stronger than the northeasterly airflow on the west side of the depression, thereby inducing the depression to move farther northeastward [9]. This was due to the fact that Yagi moved along the edge of the subtropical high. An obvious low-level southerly jet stream existed on the edge of the subtropical high, thus making the wind speed on the east side of tropical depression Yagi stronger than on its west side. This asymmetric structure was the most significant factor inducing Yagi to abruptly turn to the northeast. From 1200 UTC 14 August (Figure 8(b)) to 0000 UTC 15 August (Figure 8(c)), as the depression moved north, the asymmetric structure weakened, allowing Yagi to remain over the Bohai Sea for an extended period. At 1200 UTC 15 August (Figure 8(d)), the northeasterly airflow on the northwest side of Yagi strengthened, while the southerly airflow on its east side weakened, compelling the depression to return back to land, where it weakened and dissipated. Comparing Figure 8 to Figure 7, we can see that the low-level jet steam pattern was consistent with the steering flow, indicating that the low-level jet stream played an important role in the steering flow around tropical depression Yagi.

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Figure 8: 700 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The contours represent the GPH (gpm), the vectors represent the winds at 700 hPa, and the shading represents the wind speed (m/s).

In order to analyze the diagnostic features of Yagi’s development, we generated the vorticity advection at 700 hPa and the temperature advection at 850 hPa (Figure 9). At 0000 UTC 14 August (Figure 9(a)), positive vorticity advection existed on the south side of the Bohai Sea along the track of Yagi at 700 hPa, along a southwest-northeast axis. This indicated that the depression would strengthen when it move along that axis. Over the Bohai Sea at 850 hPa, weak cold advection occurred, indicating that the cold air extended to the southeast. At 1200 UTC 14 August (Figure 9(b)), the vorticity advection at 700 hPa along the track of tropical depression Yagi was not apparent, but the cold advection at 850 hPa above the Bohai Sea had strengthened. This indicated that the intensity change of Yagi was not obvious, but the cold air advection at 850 hPa had increased. At 0000 UTC 15 August (Figure 9(c)), positive vorticity advection occurred over the southern portion of the Bohai Sea, with negative vorticity advection over its northern portion. This configuration implied that tropical depression Yagi would strengthen in the southern part of the Bohai Sea and remain over the Bohai Sea for an extended period. Cold advection at 850 hPa above the Bohai Sea also occurred. At 1200 UTC 15 August (Figure 9(d)), positive vorticity advection at 700 hPa was present from the south Bohai Sea to the land, indicating that Yagi had returned back to land. Otherwise, at 850 hPa above the Bohai Sea, the cold advection was strong, indicating that the gale in the Bohai Sea at that time was primarily caused by the cold air.

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Figure 9: Vorticity advection at 700 hPa (contour: ×10⁹/s²) and temperature advection at 850 hPa (shading: ×10⁵ K/s): (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. The red dots represent the location of “Yagi.”

5. Model Simulation and Sensitivity Test

The effects of the atmospheric circulation on the development and movement of tropical depression Yagi have been discussed previously. The depression strengthened appreciably after it moved over the Bohai Sea not only because friction was reduced but also because of the combined influence of sea surface heating and moisture flux. To analyze the effects of the surface change from land to sea on the development of Yagi, we designed one control experiment (CE) and two sensitivity experiments (SE_1 and SE_2). The introduction of the CE, SE_1, and SE_2 can be found in Section 2.

The 10 m wind field comparison between the CE, SE_1, and SE_2 from 1200 UTC 14 August to 1200 UTC 15 August is shown in Figure 10. In the CE, at 1200 UTC 14 August (Figure 10(a)), the circulation center of tropical depression Yagi was over land, adjacent to the south side of the Bohai Sea. The north side of Yagi had already generated the ocean gale. At 0000 UTC 15 August (Figure 10(d)), the center of Yagi was located over the central Bohai Sea, and the intensity of the depression had strengthened significantly. The maximum surface winds over the ocean at this time exceeded 30 m/s. At 1200 UTC 15 August (Figure 10(g)), the center of tropical depression Yagi had returned to land from the south coast of the Bohai Sea, and the maximum winds over the ocean had weakened compared to the values observed at the previous time (Figure 10(d)). For the 10 m wind field of the CE, the results of the simulation were approximately the same as the observations. In SE_1, in which the sea surface was changed to cropland, the 10 m wind fields of SE_1 and the CE were almost the same at 1200 UTC 14 August (Figure 10(b)). However, at 0000 UTC 15 August (Figure 10(e)), the SE_1 wind speed was lower than the corresponding value in the CE (Figures 10(e) vs. 10(d)), and the SE_1 center of the circulation was loosely structured. At 1200 UTC 15 August (Figure 10(h)), the strong wind zone seemed larger in SE_1 than in the CE, and the cyclone center shifted more to the east. In SE_2, the sea surface heat and moisture flux were turned off in the model. At 1200 UTC 14 August (Figure 10(c)), the 10 m wind fields of SE_2 and the CE approached that of SE_1, and the location of the center of tropical depression Yagi as well as its intensity was nearly the same in SE_2, SE_1, and the CE. At 0000 UTC 15 August (Figure 10(f)), compared with SE_1 and the CE, the intensity of the center of Yagi was noticeably weaker in SE_2, with the mean wind over the sea ranging from approximately 17.2 to 24.4 m/s. At 1200 UTC 15 August (Figure 10(i)), the center of tropical depression Yagi was still located on the south side of Bohai Sea in SE_2, while it had already moved back over land in the CE and SE_1. In addition, the wind speed intensity in SE_2 was weaker than the CE and SE_1 values, ranging from approximately 13.9 to 20.7 m/s. From the comparison of SE_1, SE_2, and the CE, it can be deduced that the intensity of tropical cyclone Yagi and the wind speed intensity in the Bohai Sea weakened a small amount when the sea surface was changed to cropland and changed noticeably after the sea surface heat and moisture flux were turned off. These results indicate that the heat and moisture flux from the sea surface played a more important role in the development of Yagi than the reduction of friction when the surface changed from land to sea. The friction of the cropland surface did, however, significantly impact the breakdown of the structure of Yagi. Additionally, changes to the surface friction and surface flux also affected the track of Yagi, a finding that warrants further investigation. The sensitivity experiments revealed that the surface change from land to sea played an important role in the development and movement of tropical depression Yagi.

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Figure 10: Comparison of 10 m winds in the CE, SE_1, and SE_2. The red line represents the track of Yagi (from 37°N, 117°E, to 39°N, 122°E), the vectors represent the 10 m wind, and the shading represents the wind speed (m/s). (a–c) At 1200 UTC 14 August; (d–f) at 0000 UTC 15 August; (g–i) at 1200 UTC 15 August. (a), (d), and (g) show the CE; (b), (e), and (f) show SE_1; (c), (f), and (i) show SE_2.

Along the track of tropical depression Yagi (the red line in Figure 10(a)), the vertical cross section was generated in order to analyze the vertical spatial structure of the potential vorticity (the contours, which represent the intensity of Yagi) and the apparent heat source (the shading, which represents diabatic heating) (Figure 11).

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(i)

Figure 11: Vertical cross section of the apparent heat source Q₁ (shading: K/day) and potential vorticity (contour: PVU) along the track of Yagi (the red line in Figure 9(a)) in the CE and SEs. (a–c) At 1200 UTC 14 August; (d–f) at 0000 UTC 15 August; (g–i) at 1200 UTC 15 August. (a), (d), and (g) show the CE; (b), (e), and (h) show SE_1; (c), (f), and (i) show SE_2.

From this figure, it can be seen that, at 1200 UTC 14 August (Figures 11(a)–11(c)), the intensity of Q₁ and the potential vorticity on the vertical cross section in SE_1 (Figure 11(b)) and the CE (Figure 11(a)) are similar, while the intensity of Q₁ in SE_2 is weaker (Figure 11(c)). At this time, the center of tropical depression Yagi was still located over land, along the coast. At 0000 UTC 15 August (Figures 11(d)–11(f)), the center of Yagi had moved over the Bohai Sea (the center was located at approximately 200 km in the profile). In the CE (Figure 11(d)), the values of Q₁ below 500 hPa were strongly negative, indicating the convergence of diabatic heating at low levels and the absorbance of diabatic heat from the sea surface by Yagi. However, at the midlevels of the troposphere, Q₁ became strongly positive, indicating that the midlevel of the troposphere was the source of the diabatic heating. In other words, tropical depression Yagi released a great deal of heat through the latent heat of condensation at this height. At 0000 UTC 15 August in the CE, the potential vorticity center was strong, indicating that the intensity of the low-pressure center was strong. At the same time in SE_1 (Figure 11(e)), the negative region of Q₁ tilted a greater distance, appearing at lower heights to the right of the vortex center, revealing that the vortex absorbed diabatic heating from the surface ahead of it and released diabatic heating behind it, a configuration that would induce a vertically tilted vortex structure. The absolute values of Q₁ in the midlevels and low levels of the troposphere in SE_2 at this time (Figure 11(f)) were smaller than those in the CE and SE_1, and the SE_2 intensity of the potential vorticity was weaker. These results indicate that, after the sea surface heat and moisture flux were turned off, tropical depression Yagi absorbed less diabatic heating from the lower troposphere, leading to a weakening of its intensity and the subsequent release of less diabatic heat in the midtroposphere. By 1200 UTC 15 August (Figures 11(g)–11(i)), the center of Yagi had moved back over land in the CE and SE_1, and the released diabatic heating from this depression in the CE and SE_1 was stronger than it was in SE_2 because of the previous development intensity of Yagi.

6. Conclusions and Discussion

This study analyzed the meteorological processes that occurred as tropical depression Yagi entered the Bohai Sea and induced an ocean gale on 14-15 August 2018. At 0000 UTC 14 August, tropical cyclone No. 14, “Yagi,” weakened and lost its numerical designation from the CMA and continued moving northeastward. During the night of 14 August, Yagi entered the Bohai Sea from the south coast. Because of the decreased friction over the ocean, as well as the increased diabatic heating from the sea surface and moisture flux, the intensity of the cyclone strengthened noticeably, and the wind speed along its periphery increased sharply. Meanwhile, during the daylight hours of 15 August, there existed a cold high-pressure system at high latitudes at the surface; cold air from this system flowed over the Bohai Sea along the north coast, influenced by the local terrain. A concentrated pressure gradient zone was present on the southern periphery of the high-pressure center, which, in combination with the northwest side of TC Yagi, formed a tight pressure gradient region, which induced the ocean gale that strengthened during the daylight hours of 15 August. The combined effect of the rejuvenated tropical cyclone Yagi and the flow of cold air induced the gale in the Bohai Sea. During the evening of 15 August, as the cold air continued to push southeastward, tropical depression Yagi moved back over land, where it rapidly weakened and dissipated. Thus, the sharp increase of wind speed in the Bohai Sea when the center of Yagi moved over the water, the return of the depression to land, and the dissipation of the cyclone were all connected with the surface change from land to sea and the cold air from high latitudes.

In addition, in the upper levels of the troposphere, the long wave adjustment of the 200 hPa westerly jet at high latitudes provided favorable conditions for Yagi to continue moving northeastward and for the development of cold air in the lower troposphere via the divergence and convergence of the upper-level westerly jet. As the upper trough and ridge progressed eastward, the convergence region of the westerly jet also shifted to the east, which induced the surface cold air to move southeastward, as the divergence area weakened and disappeared. At the middle tropospheric height of 500 hPa, it can be seen that the northeastward track of tropical depression Yagi was along the periphery of the subtropical high on 14 August. The more northward location of the subtropical high was the primary factor behind the continued northeastward movement of tropical depression Yagi, a configuration that was also influenced by TC “Bebinca” in the South China Sea. During the daylight hours of 15 August, the effects of the more northward subtropical high and the cold air from high latitudes combined to induce tropical depression Yagi to remain over the Bohai Sea for an extended period of time. During the evening of 15 August, the subtropical high split, with a portion of it moving back over the ocean. Under the influence of the cold air, Yagi returned to land and dissipated. Thus, the track of Yagi was not only related to the location of the subtropical high but also connected with the formation of TC “Bebinca.” The steering flow explained the reason that the track of Yagi first curved northeastward over the sea and then returned to land. From the wind field structure in midlevels and low levels of the troposphere at 700 hPa, the northeastward movement of Yagi on 14 August was associated with the asymmetric structure of the cyclone itself as well as the low-level jet stream on the periphery of the subtropical high, which was also an important factor in the steering flow. From the analysis results, in the midlevels and low levels of the troposphere, both the vorticity advection at 700 hPa and the temperature advection at 850 hPa corresponded well with the development and movement of Yagi, as well as the behavior of the cold air.

Finally, in order to investigate the effects of friction and sea surface flux on the intensity of tropical depression Yagi, we used the WRF mesoscale model to simulate and design two sensitivity experiments. When the sea surface was changed to cropland in SE_1, the intensity of Yagi decreased and its structure became looser. After turning off the sea surface heat and moisture flux in SE_2, the intensity of Yagi and the wind speed along its periphery decreased significantly, indicating that the sea surface friction and diabatic heating from the sea surface heat and moisture flux played a key role in the intensity and structural changes of Yagi after it moved out over the ocean.

Data Availability

The platform automatic station hourly wind data and the tropical cyclone track data are provided from China National Meteorological Data Service Center (http://data.cma.cn). The ERA-Interim dataset is obtained from https://www.ecmwrf.int/en/forecasts/datasets/archive-datasets/reanalysis-dataset/era-interim. The WRF model version 3.9.1 is freely available at https://www.mmm.ucar.edu/weather-researchand-forecasting-model. The NCEP FNL data are obtained from https://rda.ucar.edu/datasets/. And the NOAA sea surface temperature data are obtained from ftp://polar.ncep.noaa.gov/pub/history/sst.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 41575049), Forecaster Special Projects of the China Meteorological Administration (CMA) (CMAYBY2019-009), and Science and Technology Collaborative Innovation Fund Projects of Circum-Bohai-Sea Region (QYXM201712 and QYXM201808). The authors thank LetPub (http://www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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